For festivals: NGOs usually do not have much money. So at public events, they are used to collecting donations via donation boxes or active fundraising on site. This may not align with your expectations, or if it is a cashless festival, won't work. Instead, think of a solution that helps both, the NGOs and you. E.g., hang posters that list all your NGOs and respective QR codes through which people can donate online.
For NGOs: At festivals, the situation will mostly be like this: you are one of many NGOs and the audience does not necessarily expect to be asked to donate. Maybe the festival is also cashless. Check with the festival operator in advance if you are able and allowed to collect donations and consider a cashless option, for example donating via QR codes. Sometimes you have to accept that festivals aren't the right place to collect donations – but to get in touch with people.
Find some inspiration for online fundraising at WhyDonate Fundraising Blogs:
https://whydonate.nl/en/blog/Or find a clever idea for donating in this news release of Roskilde Festival:
https://www.roskilde-festival.dk/en/years/2023/news/open-call-2022/
Instead of allowing cash donations, Superbloom Festival put up a “Wall of Support” in their NGO area in 2022, which showed all participating organizations with weblinks and QR codes that allowed visitors to donate online.